Boiler coating and method for applying



Jan. 10, 1961 G. J. EXTALE ETAL 2,967,365

BOILER COATING AND 'METHOD FOR APPLYING Filed July 15, 1954 [)7 van 6 ans: G'Mer't JZXa/e,

Freder/b/r W p/bd'rzgon 2y 077M"? I 7776/)" Attorney.

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BOILER COATING AND METHOD FOR APPLYING Gilbert J. Extale and Frederick W. Riddington, Ontario,

Calif asslgnors to General Electric Company, a corporation of New York Filed July 15, 1954, Set. N0. 443,644

11 Claims. (Cl. 38-77) This invention relates to boiler coatings and more parttcularly to coatings for the flash boilers of steam irons. In many present day steam irons, water is fed at a regulated rate from a reservoir to a flash boiler which is usually located in the heated soleplate of the iron. The water dropping onto the hot boiler surface is flashed into steam and then passes ultimately through steam ports in the bottom of the soleplate for assisting in the ironing operation. To iron in the most efiicient manner and to avoid any danger of spotting of the material, it is necessary that the water be completely converted into steam before passing through the steam ports. As is well known, water dropping onto a hot metallic surface has a tendency to form into globules which bounce about on the hot surface without being converted substantially instantaneously into steam. It is possible for such globules to. be entrained by the steam forming in the boiler, car ried through the passages to the steam ports, and ejected as water onto the surface being ironed.

To minimize the above tendency of the water to form into globules and to facilitate the instantaneous conversion of the water into steam, it has been the previous practice of the assignee of the present application to steam each iron after assembly and before shipment in order to secure a satisfactory coating thereon. For such steaming either a solution of gypsum or a solution including a mixture of gypsum and hydrated lime was used. This solution was supplied to the completed iron and the iron was steamed in the regular manner. As the solution dropped onto the boiler surface and the water was evaporated to steam, the gypsum or the gypsum and hydrated lime mixture deposited as a coating on the boiler While this coating reduced the tendency of the water to form into entrainable globules, it had certain disadvantages. In the first place, since the steaming operation for the purpose of coating was performed after the manufacture of the iron was complete, a great deal of the coating material was deposited on the outside of the soleplate on the bottom surface thereof in the area adjacent the steam ports. This required a bufi'ing operation to remove such deposits, and these deposits, because of the nature thereof and because of the location of a portion thereof in the steam port recesses, were rather difiicult to remove. In addition, these previous coatings were such that the iron would ordinarily start to spit at temperatures in excess of about 400 degrees F.; that is, the aforementioned globules would form and be carried out through the steam ports with the steam. Finally, the coating was non-uniform, being heavy in some places and absent in others.

It was also found in some cases that after a period of time this old coating had an occasional tendency to come out as small white flakes, being ejected through the steam ports onto the surface being ironed. This means that the coatings previously used did not in all cases have sufficient adhesion for the purpose. Finally, the coatings previously used were at least to a limited extent soluble under certain circumstances .so that if the iron were flooded United States Patent some of the coating could wash out through the steam ports, introducing the possibility of staining the material being ironed.

We have developed a steam iron with a boiler having a coating which overcomes the above disadvantages. The coating material employed in our invention can be applied to the surface of the boiler by the method of our invention before any assembly of the soleplate into the completed iron. Furthermore, the coating material is such that an iron having the boiler surface coated there with will not spit even though the iron is steamed at a temperature over 500 degrees F. Because of the fact that the coating is applied merely to the particular surface of the soleplate upon which it is desired at an early stage in the manufacture of the iron rather than being applied by a regular steaming process in the completed iron, there is no tendency for the coating to be ejected through the steam ports and deposited on the bottom of the soleplate. Therefore it is no longer necessary, as it was previously, to remove the deposited material in a difficult .butfing operation. The new coating of our invention also has a better adherence so that any tendency for small flakes to pass out through the steam ports during subsequent use improved method of application facilitates the coating of the boiler surface at an early stage in the manufacturing operation, facilitates the limiting of the deposition of the coating material to precisely the surface which it is desired to coat, and facilitates the securing of a firmly adhering, uniform, efiective coating.

Itis an object of our invention to provide an improved coating for boilers, and particularly the flash boilers, of steam irons.

It is another object of our invention to provide an improved method for securing this improved coating.

It is a further object of our invention to provide an improved coating and an improved method of applying the coating which facilitates the conversion of water into steam and minimizes any tendency of the iron to spit."

'It isstill another object of our invention to provide an improved coating for the flash boilers of steam irons and an improved method for applying suchcoating by which improved adherence to the boiler is secured and any tendency of the coating to flake ofl or to be dissolved is minimized.

Further objects and advantages of our invention will become apparent as the following description proceeds and the features of novelty which characterize our invention will be pointed out with particularity in the claims annexed to and forming part of this specification.

In carrying out the objects of our invention in one form a colloidal dispersion of hydrated silica, that is, a silica sol is employed; in other words, a liquid medium,

such as an aqueous body, in which the silica is carried i tion of the top surface of the soleplate exclusiveof the Patented Jan. 10, 1961 achieved when the material is applied in a plurality of relatively thin coats rather than in a single coat.

For a better understanding of our inventionreference may be had to the accompanying drawing in which Fig. 1 is a side elevation view, partly broken away, of a steam iron incorporating the boiler coating of our invention.

Fig. 2 is a perspective view of a soleplate being coated in accordance with the method of our invention, this figure illustrating the arrangement only diagrammatically.

Referring to Fig. 1 there is shown a steam iron 1 including a soleplate 2. The soleplate is formed with a cavity or recess 3 therein which forms a steam chamber or boiler. Heat for the soleplate and for effecting vaporization of water in the steam boiler is supplied through a heating element 4 embedded in the soleplate. The boiler is closed at the top by a cover 5 which is secured to the top of the soleplate overlying the steam chamber by a plurality of screws one of which is shown at 6. Water is supplied from a reservoir 7 through a valve 8 which is secured to the cover 5 by dome 9.

Water is supplied from the reservoir through the valve at a metered rate, dropping onto the surface 10 of the boiler 3. This surface, being heated by the heating element 4, efiects flash vaporization of the water into steam. The steam passes in a conventional manner through passages (not shown) to the steam ports one of which is indicated at 11 from which the steam is supplied to the surface of the material being ironed.

The soleplate is usually made of a metal such as aluminum. If the water is dropped onto a bare aluminum surface, there is a tendency for at least some of the water to collect in globules bouncing about on the hot surface in a well-known manner. Such globules are frequently entrained in the stream of steam passing from the boiler to the steam ports and hence are ejected as water onto the surface being ironed. This, of course, is undesirable for most satisfactory and effective steam ironing. In order to reduce the amount of such spitting it has been the practice to coat the surface 10 of the boiler in some manner.

For example, the assignee of the present application has previously followed the practice of actually steaming through the iron after manufacture was completed substantially a full reservoir of a solution which included materials such as gypsum, a mixture of gypsum and hydrated lime, etc. In the case of all of the previous coating materials with which we are familiar the practice was followed of actually steam the solution containing the coating material through the iron after final assembly was completed. During the steaming operation, of course, the water evaporated and the gypsum or the like in the solution was deposited as a coating on the surface 10 of the boiler.

These previous coating materials had numerous disadvantages. For example, steaming the solution through the completed iron required a significant amount of time, and the deposition could not be limited with certainty to the surface of the boiler which it was desired to coat. Rather some of the material carried through with the steam stream to the discharge ports and deposited on the bottom of the soleplate in the region of the discharge ports. Further some of the material deposited in the recesses of the ports themselves, for example, at 12, where it was clearly visible in the final product. It was therefore necessary to perform a rather difficult final bufiing operation to remove this material from the bottom of the soleplate and from the recesses in the region of the bottom of the soleplate.

Also it was found that the materials previously employed did not have a completely satisfactory adherence and had some tendency to flake off into small pieces which were in time carried through with the steam during use and discharged through the steam ports. Further if the iron became flooded these previous coating materials had a tendency to dissolve to some extent, the

4 dissolved material being carried out through the discharge ports, introducing the possibility of staining the material being ironed. Irons coated with the previous materials had a tendency to begin spitting at temperatures about 400 degrees F. Finally, the coating obtained was non-uniform.

To overcome the above disadvantages we provide a coating material which has improved adherence, no tendency to dissolve and which does not allow splitting even at temperatures above 500 degrees F. Moreover, it is possible to apply this new coating uniformly and easily by spraying it, for example, onto the boiler surface of the soleplate before any assembly of the iron has begun. The remainder of the top of the soleplate is conveniently masked in a suitable manner so that the deposition of the coating material is limited to the boiler surface 10. Since by this method and with this coating material the boiler coating can be secured at an early stage in manufacture, it is no longer necessary to perform the final steaming operation and then to clean the iron and particularly the soleplate by a difficult bufiing operation after this steaming has been completed.

The particular material employed in a coating in accordance with our invention is a liquid suspension which may best be described as a silica sol or a colloidal dispersion of hydrated silica. We have found that a very satisfactory composition for the purpose is a silica sol sold under the trade name Ludox. For application to the boiler surface we make a mixture of about 1 part Ludox and 6 parts demineralized water.

The material employed may be considered from a chemical standpoint as a colloidal dispersion of hydrated silica or a polymerized form of silicic acid. Ludox is an aqueous dispersion containing approximately 30% SiO with a very small amount of alkali as a stabilizing agent. An approximate chemical composition would be 30% SiO 0.3% to 0.4% Na O, and a maximum of 0.15% Na SO or other sulfate. We have found that a percentage of Na O as high as 1% may be employed. The composition employed is preferably slightly alkaline. We have found that there is a tendency toward precipitation when the pH is above 10.5. There is considerable stability even at a pH as low as 2.5. However, we have found a pH lower than 7.0 undesirable because of corrosion of equipment. Moreover, our observations indicate that a better adherence and optimum conversion of Water to steam are obtained when a slightly alkaline composition is employed. Our tests indicate an optimum pH range for use in our invention to be from 9.5 to 10.5.

The product used for the coating should be distinguished from an alkali silicate such as the sodium silicate of commerce, because, in contrast to sodium silicate, for example, it contains no significant quantity of alkali and has few properties in common with the compounds known as alkali silicates.

In practice silica sol and more specifically, by way of example, Ludox, is mixed in the approximate ratio of one part' Ludox to six parts demineralized water. Thus as actually applied, the liquid contains hydrated silicon dioxide in an approximate range of 4% to 5% in colloidal dispersion and Na O in solution in an approximate range of 0.04% to 0.15%. It is used in our normal practice at room temperature for convenience although a reasonably wide range of temperature can be employed. Also the concentration recited above is convenient for applying the material; however, this may also be widely varied within the scope of our invention.

The method of our invention is carried out in the general manner indicated diagrammatically in Fig. 2. Specifically the soleplate 2 is at least partially masked as indicated diagrammatically by 13 to cover the surface adjacent the boiler cavity 3. The surface 10 of the cavity is unshielded by the masking member so that the coating material may be applied in any suitable manner, preferably by spraying, onto this surface. The coating composition is applied to the boiler cavity 3 from a spray device indicated generally at 14.

As indicated in Fig. 2 the silica sol used for the coating material may be sprayed onto the boiler surface 10. Since the particular apparatus forms no part of this invention, a spray device has been illustrated only diagrammatically to indicate generally one way of applying the coating material.

We have found that superior results are obtainable with our method when the ultimate coat is secured by a plurality of individual thin coats applied to the boiler surface. The soleplate is heated to a temperature above the boiling point of water in order to assure the evaporation of the water from the silica sol leaving an adherent coating of silica on the boiler cavity. There is a relatively wide range of temperatures which may be employed for the soleplate, the minimum temperature being above the boiling point of water or other dispersion medium used in the coating material. In practice we actually employ a temperature of about 400 degrees F. since this facilitates the rapid evaporation of the water content of the coating composition, and insures that the soleplate will remain well above the boiling point of water throughout the coating operation. As indicated above, we prefer, and have secured the most satisfactory results by using, a plurality of relatively thin coats. Each coat is allowed to dry, or at least to set partially, before the next coat is applied. For this reason, it is advantageous to use a soleplate temperature substantially above the boiling point of water since this facilitates the evaporation of the water and the drying of each coat so that the next coat may be more quickly applied. In practice we preheat the soleplates in a conventional oven to a temperature of 500 degrees F. or more so that the soleplate will remain well above the boiling point of water throughout the application of the plurality of coats of the coating material. It will be apparent, of course, that, if desired, the soleplate could be heated in any suitable manner during application of the coating material to evaporate the liquid medium, but we have found the preheating of the soleplate to be more convenient.

In actual practice, we employ some thirty successive thin coats to secure the final coating on the boiler surface. At the temperature employed for the soleplate and with the very thin individual coats applied, there is a quick drying so that each successive coat may be very quickly applied over the preceding one. While, as indicated above, in actual practice we employ a large number of quite thin coatings, the specific number is not critical. A single coat can be employed if desired or any selected number of coats can be employed. However, as indicated above, we have found that superior results are obtained by applying a plurality of relatively thin coats rather than by attempting to secure the ultimate thickness in a single coat.

While we have shown and described a specific embodiment of our invention, we do not desire our invention to be limited to the particular form shown and described and we intend by the appended claims to cover all modifications within the spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In a steam iron, a soleplate having a boiler cavity therein for receiving water for flash conversion into steam, and an adherent coating of finely divided silica particles on the surface of said boiler cavity.

2. In a steam iron, a soleplate having a boiler cavity therein for receiving water for flash conversion into steam, and a coating on the surface of said boiler cavity, said coating comprising dried silica sol.

3. In a steam iron, a soleplate having a boiler cavity therein for receiving water for flash conversion into steam, and a coating on the surface of said boiler cavity, said coating comprising a plurality of coats of dried silica sol.

4. In a steam iron including a soleplate having a boiler cavity therein, the method of coating said cavity which comprises heating the soleplate, and applying a colloidal dispersion of hydrated silica in a liquid medium to the surface of said boiler cavity, the heated soleplate causing evaporation of the liquid medium.

5. In a steam iron including a soleplate having a boiler cavity therein, the method of coating said cavity which comprises applying to the surface of said boiler cavity a liquid composition comprising hydrated silicon dioxide in an approximate range of 4% to 5% in colloidal dispersion and Na O in solution in an approximate range from 0.04% to 0.15%, and evaporating the liquid.

6. The method of facilitating conversion of water into steam which comprises applying a colloidal dispersion of hydrated silica to the surface to be contacted by the water and causing a dried film of hydrated silica to be deposited thereon.

7. The method of facilitating flash conversion into steam of water supplied to a steam iron having a soleplate provided with a boiler cavity therein which comprises applying a. colloidal dispersion of hydrated silica to the surface of said boiler cavity and causing a dried film of said hydrated silica to be deposited uniformly thereon.

8. In a steam iron including a soleplate having a boiler cavity therein, the method of facilitating flash conversion into steam of water supplied to said cavity which comprises spraying onto the surface of said boiler cavity in a plurality of coats a colloidal dispersion of hydrated silica in a liquid medium, and evaporating the liquid medium after each coat and before the application of the next succeeding coat.

9. In a steam iron including a soleplate havinga boiler cavity therein, the method of coating said cavity which comprises heating the soleplate to a temperature in the order of 400 degrees F., and applying to the surface of said boiler cavity a colloidal dispersion of hydrated silica in a liquid medium, the heated soleplate causing evaporation of the liquid medium.

10. In a steam iron including a soleplate having a boiler cavity therein, the method of coating said cavity which comprises heating the soleplate to a temperature in the order of 400 degrees F., and applying to the surface of said boiler cavity a colloidal dispersion of hydrated silica in a liquid medium at approximately room temperature, the heated soleplate causing evaporation of the liquid medium.

11. In a steam iron including a soleplate having a boiler cavity therein, the method of coating said cavity which comprises heating the soleplate to a temperature in the order of 400 degrees F., spraying onto the surface of said boiler cavity in a plurality of individual coats a colloidal dispersion of hydrated silica in a liquid medium at approximately room temperature, the heated soleplate causing evaporation of the liquid medium after each coat and before the application of the next succeeding coat.

References Cited in the file of this patent UNITED STATES PATENTS 35 Rakos Mar. 7, 1950 2,505,629 Thomsen et a1 Apr. 25, 1950 83,320 Morton July 13, 1954 315% Thurber et a1 Oct. 26, 1954 

